Nearly 300 species of fish, mussels and other sea critters hitchhiked across the Pacific Ocean. They traveled on debris. It was from the 2011 Japanese tsunami. They washed ashore alive in the United States. That's what researchers reported Thursday.

It is the largest and longest marine migration ever documented. That's according to outside experts and the researchers. The scientists and colleagues combed the beaches. They combed beaches in Washington. In Oregon. In California. In British Columbia. In Alaska. And in Hawaii. They tracked the species to their Japanese origins. Their arrival could be a problem. That’s if the critters take root. They could push out native species. That's what the study authors said in Thursday's journal Science.

"It's a bit of what we call ecological roulette," said lead author James Carlton. He's a marine sciences professor. He works at Williams College. It is in Williamstown. It is in Massachusetts.

It will be years before scientists know if the 289 Japanese species thrive in their new home. And if they crowd out natives. The researchers roughly estimated that a million creatures traveled 4,800 miles. They went across the Pacific Ocean.

Invasive species are a major problem worldwide. It’s when plants and animals thrive in areas where they don't naturally live. Marine invasions in the past have hurt native farmed shellfish. They also eroded the local ecosystem. They caused economic losses. And they spread disease-carrying species.

A magnitude 9 earthquake off the coast of Japan triggered a tsunami. It happened on March 11, 2011. It swept boats into the Pacific. It swept docks into the Pacific. It swept buoys into the Pacific. And it swept other man-made materials into the Pacific. The debris drifted east. It drifted with an armada of living creatures. Some gave birth to new generations while at sea.

"The diversity was somewhat jaw-dropping," Carlton said. "Mollusks, sea anemones, corals, crabs, just a wide variety of species. They represented a cross-section of Japanese fauna."

The researchers collected and analyzed the debris that reached the West Coast and Hawaii. They collected it over the last five years. New pieces arrived Wednesday in Washington. The debris flowed across the North Pacific current. It moved north with the Alaska current. It moved south with the California current. Most hit Oregon and Washington.

A small boat from Japan reached Oregon. This was last year. It held 20 good-sized fish inside. It included a kind of yellowtail jack. It is native to the western Pacific. Some of the fish are still alive. They are in an Oregon aquarium. Earlier, an entire fishing ship arrived intact. It held five of the same 6-inch fish. They were swimming around inside.

Gregory Ruiz is the co-author. He is a Smithsonian marine ecologist. He is especially interested in a Japanese parasite. It is in the gills of mussels. Elsewhere in the world, these parasites have taken root. They have hurt oyster harvests. They have hurt mussel harvests. They hadn't been seen before on the West Coast.

The researchers note another huge factor in this flotilla. Plastics.

Decades ago, most of the debris would have been wood. It would have degraded over the long ocean trip. Now most of the debris are made of plastic and that survives, Carlton said. And so the hitchhikers survive, too.

"It was the plastic debris that allowed new species to survive far longer than we ever thought they would," Carlton said.

James Byers is a marine ecologist. He works at the University of Georgia. It is in Athens. He wasn't part of the study. But he praised the authors for their detective work. He said in an email that the migration was an odd mix of a natural trigger and human aspects. He said this was because of the plastics.

"The fact that communities of organisms survived out in the open ocean for long time periods (years in some cases) is amazing," he wrote.